This invention generally relates to an apparatus for decoding diphase signals and more particularly relates to an apparatus for generating clocking and data information from a diphase signal.
Various systems have been suggested for the serial transmission of data within and between the various units which comprise a communications terminal. One such system may be termed a mark-space serial signal and, still another, a diphase serial signal. The mark-space technique is efficient in that it utilizes one pair of transmission conductors; however it additionally requires some method of synchronizing the send and receive units. A diphase signal, on the other hand, transmits both data and synchronization information over a single pair of transmission conductors. The mark-space signal is a serial information stream wherein a mark is sent as a continuous on or high signal level and the space as a continuous off or low signal level. The signal remains at a constant level so long as a mark or space is present. That is, in response to a marking line a continuous high level is generated and, correspondingly, a spacing line is represented by a continuous low level. It will be appreciated that such a serial mark-space signal does not carry any clocking information. Present communication terminals generally operate from a common clock to maintain synchronization between all of the units within the system. A terminal utilizing such mark-space data signals for information transmission would usually require an additional pair of wires carrying the clocking information between the various units and thus increase the cost and complexity of the terminal interconnect system. The transmission of such serial mark-space data as well as the transmission of a clocking signal over commercial telephone lines is difficult and costly. An additional complication experienced with the mark-space type signal is its inherent inability to be coupled through isolation transformers. Such a drawback is especially important when private or commercial telephone circuits are used for the transmission of information. To overcome such restrictions, tone modulators and demodulators are often employed further increasing the system cost and complexity.
A diphase serial signal, as compared to a mark-space serial signal, provides a level transition at least once for each clock interval or data base-bit interval regardless of the data information being sent. For example, for each mark signal, the length of which is related to the clock period, a level transition will occur and thus a marking line of four mark signals will be indicated by four equally spaced signal level transitions. A space condition is indicated by an additional mid-bit transition occurring midway between the base bit transitions. For example, a space line of four space signals will be represented by eight level transitions (four base-bit transitions and four mid-bit transitions). Such diphase signals thus carry both data and clocking information. Therefore, since the signal carries both data and clock information in the form of level transitions, the separate transmission of additional clocking signals, for synchronization purposes, is unnecessary. A particular advantage with such diphase signals is that they may be transformer coupled, since all information is in the form of level transitions. Thus, the diphase signal may be transmitted over conventional telephone circuits without requiring tone modulators and demodulators.